The present invention relates to the application of a light source device including a linearly polarized light-emitting light guide body to a liquid crystal display element. More specifically, it relates to a light source device having improved utilization efficiency of light in an image displaying device using linearly polarized light by emitting the linearly polarized light of one direction, a liquid crystal display element including the light source device as a constituting element and a light guide body constituting the light source device.
A liquid crystal display device has characteristic features that it is thin in thickness and light in weight, and has small power consumption because of low voltage driving, and it is growing rapidly as a potential picture information display device.
A liquid crystal display element is generally constructed with a cell holding twisted liquid crystal between a pair of base boards and polarizing plates placed at both the sides of the cell in such a manner that the polarization axes orthogonally cross each other. An example of the polarizing plate is a dichroic polarizing plate using an oriented dichroic pigment such as a PVA-iodine system. The dichroic polarizing plate selectively absorbs only one linearly polarized light component out of a pair of polarized components orthogonally crossing each other and transmits only the other linearly polarized light component, and thereby unpolarized light is converted into linearly polarized light.
In a liquid crystal display device, unpolarized light emitted from a backlight light source is at first converted into linearly polarized light by a polarizing plate placed at the opposite side of the cell (backlight side). The converted light rotates the axis along the twist of liquid crystal molecules in the liquid crystal cell, and thereby it is not absorbed by the polarizing plate placed at this side of the liquid crystal cell (observer side) and observed as display light. When a voltage is applied on the liquid crystal cell, the liquid crystal molecules are aligned in the electric field direction with the result of disappearing of the twist of the molecules and the polarized light transmitted through the liquid crystal cell is absorbed by the polarizing plate at the observer side.
The utilization efficiency of light in a liquid crystal display device is regulated mainly by (1) the light transmittance of the polarizing plate, (2) the numerical aperture of a liquid crystal panel and (3) the light transmittance of a color filter. When the light utilization efficiency is low, the contrast (relative luminance) of image light becomes low, and as a result, the display quality becomes poor. On the other hand, when the output of the backlight light source is increased, the contrast of the image light is increased, but the power consumption is increased, and it causes a trouble of the shortening of driving time especially in the case of a portable instrument.
Further, a method for collecting light by using a prism sheet or the like has been proposed for increasing the contrast of image light, but although the contrast in the front direction is improved, the luminance in other directions is extremely lowered, and this opposes the recent trend of wide viewing angle.
The largest regulating factor in the utilization efficiency of light is the light transmittance of a polarizing plate. In the process of extracting linearly polarized light out of light-source light (unpolarized light) by the polarizing plate, theoretically 50% light or more is lost. If it becomes possible that the light-source light is converted into linearly polarized light having a vibration plane which coincides with the vibration plane of linearly polarized light which transmits through the polarizing plate, the utilization efficiency of the light will be extremely improved.
For example, U.S. Pat. No. 3,610,729 discloses a method which separates linearly polarized light of only one direction, and reflects the linearly polarized light of the direction perpendicular to the separated one and reuses the reflected one by using of an optical film prepared by laminating two kinds of films into a multilayer. Further, European Patent 606940A2 and D. J. Broer, J. A. M. M. van. Haare, G. N. Mol and F. Leenhouts; Asia Display ""95, 735 (1995) disclose methods for increasing the utilization efficiency of light by selectively transmitting circularly polarized light of only one direction, reflecting the circularly polarized light of the other direction and reusing it using a cholesteric liquid crystal and a xc2xc wave length plate.
These methods have high effect regarding conversion efficiency to polarized light or the utilization efficiency of light, but they have problems that the production of such a light source device is difficult since strict high-order structure is required, and the light source device becomes expensive.
Further, WO 92/22838 and F. M. Weber; SID 93 DIGEST, 669 (1993) disclose methods for separating polarized light by using Brewster angle. These methods can be applied at a relatively low cost, but their polarized-light conversion efficiencies are not sufficient, and further the efficiency depends largely on the emission angle of polarized light, and the kinds of the linearly polarized light to be obtained is limited.
JP-A 6-331824 (JP-A means Japanese unexamined patent publication) and JP-A 9-292530 disclose methods for separating polarized light based on the fact that the difference between refractive indexes at boundary varies depending on the direction of polarized light by using a layer having refractive index anisotropy in a light guide plate. These methods are also insufficient in polarized-light conversion efficiencies, and the utilization efficiencies of light are not high. They also have a problem that the refractive index anisotropy is limited by a material.
Further, O. A. Aphonin, et al.; Liq. Cryst., 15, 3, 395 (1993), O. A. Aphonin; Liq. Cryst., 19, 4, 469 (1995), JP-A 8-76114 and JP-A 9-274108 disclose methods using an anisotropic scatterer, in which liquid crystal is aligned by stretching a composite of a polymer and a liquid crystal, as a scatter-type polarizing plate. Furthermore, WO97/32222, WO97/32224, WO97/32226, WO97/32227, U.S. Pat. No. 5,867,316, H. Yagt, et al.; Adv. Mater., 10, 2, 934 (1998) and M. Miyatake, et al.; IDW ""98, 247 (1998) disclose methods also for obtaining a scatter-type polarizing plate by stretching an incompatible polymer blend film.
Further, JP-A 9-297204 discloses an anisotropic scatter element consisting of a stretched film in which titanium oxide having an aspect ratio of 1 or more as a component exhibiting anisotropic scattering is aligned in one direction. It describes that the light becomes the most dark when the polarization axis and the scattering axis (stretched direction) come to coincide with each other by rotating a polarizing plate on the element, and the light becomes the most bright when they become perpendicular to each other (the polarization axis and the transmitting axis come to coincide with each other).
These technologies use a method for separating polarized light by passing the polarized light of the direction (transmission axis) in which the refractive index of titanium dioxide particles and the refractive index of the polymer come to coincide with each other due to stretching or the like, and scattering backward the polarized light of the direction (scattering axis) in which the refractive indexes are not coincident with each other, that is, a method using a so-called scatter-type polarizing plate. The theory of this polarized light separation is fundamentally different from that of the objective light source device of the present invention. In the case of these technologies, since the polarized light of the direction of scattering axis must be scattered backward without scattering forward, it is required to have multiple scattering or the like by increasing scattering factors, and as a result, it becomes difficult to keep the transmittance in the direction of transmitting axis at high level. Further, in order to improve luminance, backward scattering light must be depolarized and reused; however, in the case of the scatter-type polarizing plate, a reusing ratio of light is low since the amount of dissipation light is large due to scattering, and the improving effect of luminance is low compared with a polarization degree.
WO97/32222 describes an optical film (scatter-type polarizing film) separating polarized light by using transmission/non-transmission due to scattering. The linearly polarized light of the scattering axis direction is made non-transmitting due to backward scattering by using the optical film, and the linearly polarized light of the direction of the transmission axis is transmitted, and as the result, the polarized light is separated. Therefore, in order to increase the capability of polarized-light separation, it is required to make the difference of transmittances as large as possible; ideally TTmax greater than  greater than TTminxcx9c0. The publication describes that the optical film has a diffusion reflectivity of 30% or more in the scattering axis direction.
The object of the present invention is to provide a new liquid crystal display element including a light source device converting unpolarized light into linearly polarized light.
Another object of the present invention is to provide a new light source device having high light utilization efficiency suitable for the above liquid crystal display element.
Still another object of the present invention is to provide a new light guide body constituting the above light source device of the present invention.
Yet another object of the present invention is to provide a light guide body emitting linearly polarized light converted out of the unpolarized light introduced from a light source.
Yet further object of the present invention is to provide the application of a haze anisotropic film in a light guide body.
The inventors of the present invention pursued zealous study, and as a result, they found that when a haze anisotropic layer is placed on a surface of a light guide body, light containing a large amount of linearly polarized light components in one vibration direction is emitted, and the vibration direction is coincident with the scattering axis of such a haze anisotropic layer, that is, the direction where the haze is the largest. The present invention is completed based on such finding.
Further, the light source device of the present invention was provided for the first time base on the following fact which had been found first by the inventors of the present invention: the emitted light is mainly composed of forward scattered light of specific linearly polarized light; it isbasically free from backward scattered light and transmitted light; and the vibration plane of the emitted linearly polarized light can be selected by the light scattering property of a haze anisotropic layer.
In the present invention, the above-mentioned objects and advantageous points of the present invention are attained by a liquid crystal display element including
(I) a light source device comprising the following (i) to (iii):
(i) a light guide body comprising a transparent medium, and having an end face through which the light emitted from a light source can be introduced, a pair of opposing faces one of which works as a light emission face and a haze anisotropic layer, wherein the haze value is expressed by the following formula (1) when an incident light is lineally polarized light,
H(%)=DF/TTxc3x97100xe2x80x83xe2x80x83(1)
xe2x80x83here, DF is a diffused light transmittance, and TT is a total light transmittance;
(ii) a light source placed at the end face of said light guide body, and
(iii) a reflecting body placed at the opposite side of the light emission face side of said light guide body, and
(II) a polarizing plate placed at the light emission face side of the said light guide body in such a manner that the polarization axis is parallel to the direction having the largest haze value in said haze anisotropic layer.